1. Field of the Invention
The present invention relates to a light-emitting device using an organic light-emitting layer capable of emitting light by electrically pumping or the like and having a two-dimensional photonic band gap structure.
2. Related Art
Semiconductor light-emitting devices utilizing photonic crystals are being studied in recent years (for example, Japanese Patent Application Laid-open No. 9-232669). These types of semiconductor light-emitting devices are expected to produce a resonator which can firmly confine light within crystals and to provide coherent light at an extremely high efficiency.
However, when a semiconductor is used, a boundary area of unit medium layers (each one unit in the periodical structure) becomes irregular and is affected by impurities because in semiconductors the unit medium layers are formed by crystals. It is thus difficult to obtain a uniform periodical structure and to produce a high performance light-emitting device having superior characteristics as photonic crystals. In addition, when a semiconductor is used, there is a limitation to the combination of materials with different refractive indices.
An object of the present invention is to provide a light-emitting device which can produce light with a narrow spectrum width with remarkably high efficiency and can be manufactured from an organic light-emitting material utilizing a two-dimensional photonic band gap.
The light-emitting device as the first aspect of the present invention comprises:
an optical section having a two-dimensional periodical refractive index distribution and capable of forming a photonic band gap;
a defect section formed on part of the optical section and designed so that the energy level caused by the defect is within a prescribed emission spectrum; and
an organic light-emitting layer.
The light-emitting device has an organic light-emitting layer capable of emitting light by electrically pumping or optical pumping. For example, when the electrically pumping is used, electrons and holes are injected into the organic light-emitting layer respectively from a pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by recombination of the electrons and holes in the organic light-emitting layer. At this time, light with a wavelength in the photonic band gap of the above optical section cannot be transmitted through the optical section. Only the light with a wavelength equivalent to the energy level caused by the defects can be transmitted through the optical section. Therefore, light with a very narrow emission spectrum width with inhibited spontaneous emission in two dimensions can be obtained at high efficiency by specifying the width of energy level caused by the defect.
Any materials can be used for the optical section in the present invention insofar as the materials have a two-dimensional periodical refractive index distribution and are capable of forming a photonic band gap. The optical section may have a structure such as a grating-shaped structure, a multi-layer structure, a column or other columnar-shaped structure, or combinations of these structures.
The defect section of the organic light-emitting layer and the optical section may have the following configurations.
(1) The organic light-emitting layer formed in the defect section also functions as the defect section.
(2) The organic light-emitting layer also functions as part of the defect section and as one type of medium layer of the optical section.
More particularly, the light-emitting device may have the following structure.
(A) The light-emitting device as the second aspect of the invention comprises:
a first optical section having a periodical refractive index distribution in a first direction and being capable of forming a photonic band gap;
a second optical section having, a periodical refractive index distribution in a second direction which is perpendicular to a first direction, the second optical section being capable of forming a photonic band gap; and
a defect section formed in at least one of the first and second optical sections and designed so that the energy level caused by the defect is within a prescribed emission spectrum; and
an organic light-emitting layer.
Light with a very narrow emission spectrum width with inhibited spontaneous emission in two-dimensions can be obtained at high efficiency by the combination of the first optical section which inhibits propagation of light in a first direction (X direction) and the second optical section which inhibits propagation of light in a second direction (Y direction).
(B) The light-emitting device as the third aspect of the invention comprises:
an optical section having a periodical refractive index distribution in first and second directions and capable of forming a two-dimensional photonic band gap;
a defect section formed on the optical section and designed so that the energy level caused by the defect is within a prescribed emission spectrum; and
an organic light-emitting layer,
wherein the optical section includes columnar-shaped first medium layers arranged in a square lattice shape and second medium layers formed between the first medium layers.
light with a very narrow emission spectrum width with inhibited spontaneous emission in two dimensions and two-directions can be obtained at high efficiency by the columnar-shaped first medium layers arranged in a square lattice shape and second medium layers formed between the first medium layers.
(C) The light-emitting device as the fourth aspect of the invention comprises:
an optical section having a periodical refractive index distribution in first, second and third directions and capable of forming a two-dimensional photonic band gap;
a defect section formed on part of the optical section which is designed so that the energy level caused by the defect is within a prescribed emission spectrum; and
an organic light-emitting layer.
light with a very narrow emission spectrum width with inhibited spontaneous emission in two dimensions and three-directions can be obtained at high efficiency by the optical section having a periodical refractive index distribution in first, second and third directions and capable of forming a two-dimensional photonic band gap, such as an optical section including columnar-shaped first medium layers arranged in a triangular lattice or a honey-comb lattice and second medium layers formed between the first medium layers.
(D) The light-emitting device as the fifth aspect of the invention comprises:
an optical section having a concentric and periodical refractive index distribution and capable of forming a two-dimensional photonic band gap;
a defect section formed on the optical section and designed so that the energy level caused by the defect is within a prescribed emission spectrum; and
an organic light-emitting layer,
wherein the optical section includes columnar-shaped first medium layers arranged regularly and second medium layers formed between the first medium layers.
This structure of the optical section inhibits spontaneous emission in the directions of two dimensions.
In the above described light-emitting device the organic light-emitting layer has materials which can emit light by electrically pumping and the light-emitting device may comprise a pair of electrode layers for applying an electric field to the organic light-emitting layer.
Preferably, the light-emitting device in these aspects of the invention further comprises at least one of a hole transport layer or an electron transport layer.
The use of an organic light-emitting layer has the following advantages over the case in which the photonic band gap is formed by a semiconductor. Specifically, the light-emitting device comprising the organic light-emitting layer is less affected by the irregular state and impurities of the boundary area of the light-emitting layer than the case of using semiconductors, whereby excellent characteristics from the photonic band gap can be obtained. Furthermore, in the case of forming a medium layer from an organic layer, the manufacture becomes easy and a periodic structure with an effective refractive index can be obtained, whereby superior photonic band gap characteristics can be obtained.
Some of the materials which can be used to form each section of the light-emitting device according to the present invention will be illustrated below. These materials are only some of the conventional materials. Materials other than these materials can also be used.
(Organic Light-emitting Layer)
Materials for the organic light-emitting layer are selected from conventional compounds to obtain light with a designed wavelength.
As examples of such organic compounds, aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazole dimers (OXD-8), distyrarylene derivatives (DSA), beryllium-benzoquinolinol complex (Bebq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, polyphyrin zinc complex, benzooxazole zinc complex, and phenanthroline europium complex which are disclosed in Japanese Patent Application Laid-open No. 10-153967 can be given.
Specific examples of materials for the organic light-emitting layer include compounds disclosed in Japanese Patent Application Laid-open No. 63-70257, No. 63-175860, No. 2-135361, No. 2-135359, No. 2-152184, No. 8-248276, and No. 10-153967. These compounds can be used either individually or in combinations of two or more.
(Optical Section)
Conventional inorganic and organic materials can be used for the medium layers of optical sections.
Typical examples of inorganic materials include TiO2, TiO2xe2x80x94SiO2 mixture, ZnO, Nb2O5, Si3N4, Ta2O5, HfO2, and ZrO2 disclosed in Japanese Patent Application Laid-open No. 5-273427.
Typical examples of organic materials include various conventional resins such as thermoplastic resins, thermosetting resins, and photocurable resins. These resins are appropriately selected depending on a method of forming layers and the like. For example, in the case of using a resin which can be cured by energy of at least either heat or light, commonly used exposure devices, baking ovens, hot plates, and the like can be utilized.
As examples of such materials, a UV-curable resin disclosed in Japanese Patent Application No. 10-279439 applied by the applicant of the present invention can be given. Acrylic resins are suitable as such UV-curable resins. UV-curable acrylic resins having excellent transparency and capable of curing in a short period of time can be produced using various commercially-available resins and photosensitizers.
As specific examples of basic components of such UV-curable acrylic resins, prepolymers, oligomers, and monomers can be given.
Examples of prepolymers or oligomers include acrylates such as epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, and spiroacetal-type acrylates, methacrylates such as epoxy methacrylates, urethane methacrylates, polyester methacrylates, and polyether methacrylates, and the like.
Examples of monomers include monofunctional monomers such as 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, N-vinyl-2-pyrrolidone, carbitol acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, dicyclopentenyl acrylate, and 1,3-butanediol acrylate, bifunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, and pentaerythritol diacrylate, and polyfunctional monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate.
Although inorganic and organic materials which form the media for optical sections are given above, when an organic light-emitting layer also functions as a medium layer, materials forming such an organic light-emitting layer can also be used as the medium layer.
(Hole Transport Layer)
As materials for the hole transport layer which is optionally formed, materials conventionally used as hole injection materials for photoconductive materials or a hole injection layer for organic light-emitting devices can be selectively used. As the materials for the hole transport layer, any organic or inorganic materials which have a function of either hole introduction or electron barrier characteristics are used. Materials disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such materials.
(Electron Transport Layer)
Materials for the electron transport layer which is optionally formed are only required to transport electrons injected from the cathode to the organic light-emitting layer and can be selected from conventional materials. Materials disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such substances.
(Electrode Layer)
As the cathode which is optionally provided, electron injectable metals, alloys, electrically conductive compounds with a small work function (for example, 4 eV or less), or mixtures thereof can be used. Materials disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such electrode materials.
As the anode which is optionally provided, metals, alloys, electrically conductive compounds with a large work function (for example, 4 eV or more), or mixtures thereof can be used. In the case of using optically transparent materials as the anode, transparent conductive materials such as CuI, ITO, SnO2, and ZnO can be used. In the case where transparency is not necessary, metals such as gold can be used.
The properties (such as the refractive index) and shape of medium layers, as well as the pitch, number, and aspect ratio of grating and columnar-shaped parts, are appropriately adjusted so that the optical section forms photonic band gaps.
The optical section can be formed by conventional methods without specific limitations. Typical examples of such methods are given below.
(1) Lithographic Method
In this method, a positive or negative resist is irradiated with ultraviolet rays, X-rays, or the like. The resist layer is patterned by development to form an optical section. As a patterning technology using a polymethyl methacrylate resist or a novolak resin resist, for example, technologies disclosed in Japanese Patent Applications Laid-open No. 6-224115 and No. 7-20637 can be given.
As a technology of patterning polyimide by photolithography, for example, technologies disclosed in Japanese Patent Applications Laid-open No. 7-181689 and No. 1-221741 can be given. Furthermore, Japanese Patent Application Laid-open No. 10-59743 discloses a technology of forming an optical section of polymethyl methacrylate or titanium oxide on a glass substrate utilizing laser ablation.
(2) Formation of Refractive Index Distribution by Irradiation
In this method, the optical waveguide section of the optical waveguide is irradiated with light having a wavelength which produces changes in the refractive index to periodically form areas having a different refractive indices on the optical waveguide section, thereby forming an optical section. As such a method, it is preferable to form an optical section by forming a layer of polymers or polymer precursors and polymerizing part of the polymer layer by irradiation or the like to periodically form areas having a different refractive index. Such a technology is disclosed in Japanese Patent Applications Laid-open No. 9-311238, No. 9-178901, No. 8-15506, No. 5-297202, No. 5-32523, No. 5-39480, No. 9-211728, No. 10-26702, No. 10-8300, and No. 2-51101.
(3) Stamping Method
An optical section is formed by, for example, hot stamping using a thermoplastic resin (Japanese Patent Application Laid-open No. 6-201907), stamping using an UV curable resin (Japanese Patent Application No. 10-279439), or stamping using an electron-beam curable resin (Japanese Patent Application Laid-open No. 7-235075).
(4) Etching Method
A thin film is selectively patterned using lithography and etching technologies to form an optical section.
Methods of forming an optical section have been described above. In summary, the optical section consists of at least two areas, each having a different refractive index, and can be fabricated, for example, by a method of forming the two areas from two materials having a different refractive index, a method of forming the two areas from one material and modifying the material forming one of the two areas so that the two areas have a different refractive index, and the like.
Each layer of the light-emitting device can be formed by a conventional method. For example, the organic light-emitting layer is formed by a suitable film-forming method depending on the materials. A vapor deposition method, spin coating method, LB method, ink jet method, and the like can be can be given as specific examples.